The invention relates to a method of cleaving a plate of brittle material, in which a thermal load is applied to at least one major surface of a plate to be cleft, one side of which plate is provided with a crack initiation which serves as the beginning of a crack. As a result the application of the thermal load stresses are produced in the plate, which, to be formed, cause a crack at right angles to the major surface along a desired cutting line and starting from the crack initiation.
A method of the kind to which the present invention relates is known from DE-28,13,302 C2. In this known method, a side of a plate of flat glass is provided with a crack initiation and the plate is heated in one of two consecutively arranged regions symmetrical to the desired cutting line and is cooled in the other of these regions, as a result of which due to temperature gradients thermal stresses are produced in the plate, which cause a crack starting from the crack initiation. When the plate is cleft symmetrically, the crack follows the desired cutting line. However, if the plate is cleft asymmetrically, the crack deviates from the desired cutting line.